This disclosure relates to the field of conductive polymer electronic components and devices. In particular, it relates to resistive devices comprising a layer of thermally-sensitive resistive material, such as a conductive polymer, that is laminated between a pair of planar electrodes, wherein the device has a surface-mountable configuration.
Conductive polymer thermally-sensitive resistive devices have become commonplace on electronic circuits. These include devices that exhibit a positive temperature coefficient of resistivity (PTC) and a negative temperature coefficient of resistivity (NTC). In particular, resistive devices comprising a conductive polymer resistive material exhibiting a positive temperature coefficient of resistivity (PTC) have found widespread uses as over-current protection devices or “self-resettable fuses,” due to their ability to undergo a rapid and drastic (at least three or four orders of magnitude) increase in resistance in response to an over-current situation.
It is a common design goal for electronic components to reduce the surface area or “footprint” that they occupy on a circuit board, so that circuit boards can be made as small as possible, and so that component density on a circuit board of a specific area can be increased. One way of achieving a compact geometry, while also achieving economies in manufacturing costs, is to configure the components to be “surface-mountable” on a circuit board. A surface-mountable component is flush-mounted on conductive terminal pads on the board, without the need for sockets or through-board pins.
Various surface-mountable configurations have been devised for conductive polymer thermal-resistive devices, particularly PTC devices. There are several design criteria in making surface-mountable conductive polymer PTC devices, besides the criterion of having a small footprint. For example, the design of the devices most lend itself to low manufacturing costs. Furthermore, the design must provide for integrity of the connections between the metallic elements (electrodes and terminals) and the non-metallic (polymer) element(s). In many cases, the design is a compromise among these various criteria.
One problem with surface-mountable conductive polymer devices is that the metal elements tend to impose a physical constraint on the thermal expansion of the polymeric element(s) when they experience an over-current situation. Conductive polymer PTC elements are typically formed from an organic polymer, such as polyethylene, into which is mixed conductive particles, such as carbon black or metallic particles. The conductivity (or, conversely, the resistivity) of the composition is determined, in substantial part, by the average spacing between the conductive particles. The drastic and sudden increase in resistivity of a conductive polymer element in a PTC device upon experiencing an over-current condition is due to a thermally-induced expansion of the polymer element, which increases the average spacing between the conductive particles within the polymeric material. To the extent that the metallic elements of such a device impose physical constraints on the expansion of the conductive polymer element(s), the functionality of the device may be impaired, especially after repeated over-current “trippings.” For example, “repeatability” (the characteristic of the device to exhibit substantially the same operational parameters) may degrade over a multitude of duty cycles (over-current tripping and subsequent resetting upon removal of the overvoltage), due to a kind of stress-induced “hysteresis” effect.
In particular, typical prior art conductive polymer PTC devices tend to exhibit poor resistance stability as a function of the number of duty cycles. This means that the normal (non-over-current condition) resistance in many prior art conductive polymer PTC devices tends to increase markedly after as few as 40-50 duty cycles. Furthermore, to the extent that the metal elements allow at least some degree of polymeric expansion, the metal elements are subject to mechanical stresses that may compromise the physical integrity of the device over repeated duty cycles.
Thus, there has been a long-felt, but as yet unsatisfied, need for a surface-mountable conductive polymer resistive device, particularly a PTC device, that is economical to manufacture, that has a small circuit board footprint, and that allows adequate thermal expansion of the polymer element without subjecting the metal elements to undue stress.